Cellular immunity in HIV-1 infection has been an area of intense interest in studies of pathogenesis and vaccine development. HIV-1-specific CD8+ cytotoxic T lymphocytes (CTL) are believed to have an important antiviral effector role, thus many vaccine development efforts have focused on eliciting CTL responses. However, several issues concerning HIV-1-specific CTL and their overall failure remain poorly understood. In most infected persons, CTL fail to control infection and prevent eventual disease progression. The mechanisms of this failure and HIV-1 persistence in the face of vigorous CTL responses are not known, but likely include function(s) of the Nef protein (which is believed to interfere with infected cell clearance through HLA downregulation). Intriguing data from humans and macaques infected with Nef-defective HIV-1 and SIV reveal a central role for Nef for disease development. Nef is not directly required for HIV-1 replication, but has been implicated in a vast and bewildering array of cellular effects. Downmodulation of the HLA molecule is one of the best documented;in the prior funding period we have shown that this function is functionally significant for viral evasion of CTL and that this mechanism is a reason that HIV-1 maintains Nef despite strong immune selective pressure against this protein that is not required for viral replication. Continuing this project, we want to further explore the interactions of Nef with HIV-1-specific CTL. Important questions remain about the degree to which Nef affects CTL function and the functional constraints on Nef mutation. Understanding these issues may allow better vaccine design and/or provide a novel therapeutic avenue for HIV-1. Specifically, we plan: 1. To determine the role of epitope targeting in CTL susceptibility to Nef-mediated HLA downregulation;2. To delineate in detail the kinetics of Nef-mediated HLA downregulation versus CTL clearance of acutely HIV-1-infected cells;and 3. To screen for small molecule inhibitors of Nef-mediated HLA downregulation and other functions.